Aluminium alloy and extrusion

ABSTRACT

A population of extrusion billets has a specification such that every billet is of an alloy of composition (in wt. %): Fe &lt;0.35; Si 0.20-0.6; Mn &lt;0.10; Mg 0.25-0.9; Cu &lt;0.015; Ti &lt;0.10; Cr &lt;0.10; Zn &lt;0.03; balance Al of commercial purity. After ageing to T5 or T6 temper, extruded sections can be etched and anodized to give extruded matt anodized sections having improved properties.

[0001] Extruded matte anodised sections are made in large quantities forarchitectural and other use. The aluminium alloys used are 6000 seriesalloys in the Aluminum Association Register. This invention is concernedwith the compositions of the alloys.

[0002] The standard production method involves extruding a billet of thechosen alloy, subjecting the extruded section to an alkaline etch, andanodising the resulting matte surface. Studies on the effect ofcomposition on matte etching response have been published and each shows-the importance of microstructural features on surface quality.Typically it has been demonstrated that constituent particles,dispersoids and ageing precipitates influence the surface evolution andfinal appearance. The effects of solid solution content, with theexception of Zn additions, have largely been ignored.

[0003] However, there are three important aspects of the aluminiumextrudate etching process. Firstly, the extrudate surface finish fromthe press influences the uniformity of appearance, with a good finishrequiring less metal removal to achieve acceptable quality. Secondly,the metal removal rate controls the amount of effluent that has to becontrolled and disposed of; hence there are significant environmentalbenefits to reducing the amount of metal removed for an acceptablefinish. Thirdly, the final matte appearance of the extrudate is criticaland this means both low gloss (i.e. low brightness) and uniformity.

[0004] The benefit of high Fe content to matte finish has beenrecognised for some time and this has traditionally been used by thealuminium and metal finishing industries. The drawbacks of a high Fecontent are: 1) the extrudate surface roughness increases making theproduct incompatible with others being produced on the same press wheremill finish is critical and 2) a higher metal removal is needed toattain a uniform etched finish.

[0005] This invention results from the inventors' discovery that controlof the Cu content, and to a lesser degree also the Cr content, of thealloy can have a beneficial effect. The invention thus provides apopulation of billets resulting from more than one cast of metal havinga specification such that every billet has a composition (in wt %):Constituent Range Preferred Fe <0.35 0.16-0.35 Si 0.20-0.6 0.4-0.6 Mn<0.10 0.01-0.05 Mg 0.25-0.9 0.35-0.6  Cu <0.015 <0.010 Ti <0.10  <0.05 Cr <0.10  <0.09  Zn <0.03  <0.03 

[0006] A cast is defined as the process of converting a body of moltenmetal into a plurality of billets—often several hundred billets—of solidmetal. The body of molten metal has a composition which is controlled tofall within a predetermined specification and which is generally givento the purchaser or user of the billet. The specification is maintainedfor more than one cast, generally for a whole series of casts. In thepresent invention the specification (which is not defined herein) issuch that every billet has a composition within the ranges given above.A population of billets is an unspecified number, usually at least 50and generally much more than 100, of billets resulting from more thanone cast, usually a series of at least 5 and often more than 100 casts,of metal within the specification. A population according to theinvention would not be expected to contain any billet having acomposition outside the stated range.

[0007] Extruded sections are made by extruding billets taken from thepopulation. Preferably the extruded sections are subjected to analkaline etch and are then anodised. The invention also includesextruded sections so made.

[0008] The above alloys are within the 6000 series of the AluminumAssociation classification and are related to AA6060 and AA6063generally used to make extruded matt anodised sections.

[0009] Mg and Si combine to form dispersed Mg₂Si particles whichcontribute to dispersion strengthening of the extruded sections. If Mgor Si concentrations fall below the stated ranges, then extrudedsections may not achieve desired mechanical properties in the T5 or T6temper. When the extruded sections are subjected to alkaline etch, theMg₂Si particles are preferentially dissolved. To some extent, this isadvantageous in enhancing the desired mattening effect. But if the Mgand Si contents are too high, problems may arise with regard to ease ofextrusion and surface quality obtainable. In some circumstances it ispreferable that the Mg content be in the range 0.35-0.45%.

[0010] Fe is a preferred constituent of the alloy, partly because itcontributes to the desired mattening effect and partly because alloyscontaining no Fe are much more expensive. When the Fe content is toohigh, problems arise as discussed above.

[0011] Mn is beneficial to the desired etch response and helps tocounteract Fe by reducing pitting activity. Zn is notorious for theproduction of a bright spangled appearance. At high concentrations, Tican give rise to streaking.

[0012] The level of Cu is controlled to be less than 0.015%, preferablyless than 0.010%. As the experimental data below show, higher levels ofCu have a detrimental effect on matte finish and increase the rate ofmetal removal during etching. These very low Cu levels cannot beconsistently achieved without positive and deliberate control over alloycomposition.

[0013] The level of Cr is kept below 0.10% as is conventional. But anaddition of Cr at a level of 0.03-0.09% may be made. As the experimentaldata below show, Cr at these levels enhances the matte response toetching but without increasing the metal removal rate.

[0014] The balance of the alloy is aluminium of commercial purity. Thiswill normally be primary Al from a smelter, since it would not be easyto achieve tight compositional control of secondary Al from scrap. Theinvention is concerned with commercial scale production, and not withlaboratory experiments using high purity samples.

[0015] In performing the invention method, an Al alloy of chosencomposition is cast into a billet which is optionally homogenised andextruded into a section which is cooled. Homogenising conditions do notappear to have any material effect on the development of a mattesurface. The extruded section may be cooled in still air or morepreferably by forced air cooling or quenching.

[0016] The extruded section is preferably aged e.g. to T5 or T6 temper.This may be effected by heating the section at 150-200° C. for a time todevelop peak strength. A preferred regime is 170-185° C. for 5-6 hours.Ageing has a material effect on mattness. It is believed that ageinggrows Mg₂Si particles and that these dissolve during alkaline etch togive a matt finish.

[0017] The extruded section is subjected to alkaline etching to developa matte surface. Mention may be made of two commercially available etchsystems:

[0018] Long-life etch is mainly used in Europe and North America, andinvolves treatment for 5-20 minutes with a solution of

[0019] 100 g/l NaOH

[0020] 100-160 g/l Al ion

[0021] 30-50 g/l sequesterant e.g. Na gluconate or Na heptonate at50-75° C. This typically results in 100 g/m² metal removal.

[0022] Recovery etch is mainly used in Japan and Canada, and involvestreatment for 1-10 minutes with a solution of

[0023] 30 g/l NaOH

[0024] 50 g/l Al ion

[0025] at 50-75° C. The weaker etch solution and shorter etch timeresults in a lower level of metal removal.

[0026] After etching, the extruded section has a matte surface. Althoughmattness is generally understood as the opposite of glossiness, itsmeasurement is somewhat problematic and does vary substantiallydepending on the nature of the surface and of the treatment it issubjected to. Mattness may be measured by the test in BS 6161 at 60°. Asa rough guideline, an Al surface that has been subjected to a long lifeetch may be regarded as matte if it has a gloss value below about 100;and an Al surface that has been subjected to a recovery etch may beregarded as matte if it has a gloss value below about 150.

[0027] Then the extruded and etched section is anodised under conditionswhich may be conventional and which form no part of this invention.

[0028] The ability to control etching response is important to ingotproducers, extruders and finishers. The knowledge that two keyparameters (Cu and Cr) have such a large influence is surprising. Armedwith this knowledge, an ingot producer can control performancedownstream when other factors are beyond its control.

[0029] Reference is directed to the accompanying drawings in which:

[0030]FIG. 1 is a graph of gloss measurement (BS 6161 at 60°) vs etchtime for various alloys.

[0031]FIG. 2 is a graph of weight loss against etch time for the samealloys.

[0032]FIG. 3 is a bar chart showing the effect of composition andprocessing on matte response of various AA 6060 alloys after 12 minutesetch.

[0033]FIG. 4 is a bar chart showing the effect of composition andprocessing on weight loss of the same M 6060 alloys after 12 minutesetch.

[0034]FIG. 5 is a graph of 600 gloss against weight loss, and shows theeffect of alloying additions on the same forced air cooled A A 6060alloys.

[0035]FIG. 6 is a graph of 600 gloss against weight loss, showing theeffect of chromium level and cooling rate on gloss of the same AA 6060alloys.

[0036]FIG. 7 is a graph of gloss against weight loss.

[0037]FIG. 8 is a graph of gloss against copper content.

[0038]FIG. 9 is a graph of metal removal rate against copper content.

[0039]FIG. 10 is a graph of gloss against metal removal rate.

[0040]FIG. 11 is a bar chart comparing two different alloys under avariety of conditions.

[0041]FIG. 12 is a bar chart comparing gloss of two different alloysunder different conditions.

[0042]FIG. 13 is a graph showing the effect of ageing practice on glossand tensile strength.

EXAMPLE 1

[0043] A series of alloys has been assessed by laboratory trials usingcommercial size dc ingot, a small extrusion press and controlled etchingpractices that simulate long life and recovery type caustic etches. Thealloys had the composition (in wt %):

[0044] Si 0.45%

[0045] Fe 0.25%

[0046] Mg 0.41%

[0047] Zn 0.016%

[0048] Cr, Cu, Mn, each 0.001% unless stated

[0049] Balance commercial purity Al.

[0050] The results are shown in FIGS. 1 and 2. From a range of alloyingconditions, all at 0.08%, the Cu in solid solution was seen to havedetrimental effect on matte finish (FIG. 1) and to increase the metalremoval rate by approximately 30% (FIG. 2).

EXAMPLE 2

[0051] The alloys used in this study are set out in the Table below.Each alloy was dc cast into an ingot which was homogenised.Homogenisation was at 585° C. for two hours, in all cases except whereindicated in FIGS. 3 and 4, where one ingot was homogenised at 530° C.for 30 minutes. The homogenised ingots were extruded to form extrudedsections which were either still air cooled (1.25° C./s) or forced aircooled (6.5° C./s) and aged for 5 hours at 185° C. The extruded and agedsections were subjected to a long-life etch for 12 minutes at 60° C. Theresults of this trial are shown in FIGS. 3 and 4. As can be seen:

[0052] Homogenisation conditions have very little effect on either glossor weight loss.

[0053] Forced air cooling has a minor but beneficial effect on mattness.

[0054] Cu at 0.03% has a major and detrimental effect, both on mattness(i.e. the etched product was more glossy) and on metal weight loss.

[0055] It is not known why the etching behaviour of these alloys is sosensitive to Cu level in solid solution when all previous work hasindicated that the main parameters in the microstructure are coarse andfine particles. It is not envisaged that the Cu will play a part in theformation of dispersoids or ageing precipitates and thus must be insolid solution. One clue to the importance of solute elements can begained from the general observation that the fine scale matrix attackdominates the etched surface. Sample Alloy Composition % No. Si Fe Cu MnMg Cr Ti 1 .45 .21 .001 .002 .41 <.001   .011 2 .30 .19 .001 .002 .42<.001   0.14  3 .60 .20 .001 .002 .42 .001 .007 4 .45 .20 .001 .071 .41.001 .011 5 .43 .20 .030 .002 .40 .001 .011 6 .45 .20 .001 .002 .40 .050.010 7 .45 .20 .001 .002 .40 .103 .011

EXAMPLE 3

[0056] This example is based on the same trial and the same alloys asExample 2, but focuses on the effect of Cr. FIG. 5 shows that an alloycontaining 0.05% added chromium gives a lower gloss level for a givenmetal weight loss in comparison with a base line alloy 6060 with noadditional elements. Also included for comparison are the Cu variant, aMn addition, and an alloy containing 0.10% Cr which does not have thesame beneficial effect (samples 5, 4 and 7 in the Table).

[0057] The difference between 0.05 and 0.10% Cr is also shown in FIG. 6,but in this case the effect of cooling rate after extrusion is alsoincluded. These data indicate that process conditions are important andthis may well be linked to the precipitation of Mg₂Si on to dispersoidswhen the cooling rate is too low.

[0058]FIG. 7 is a graph of gloss against weight loss showing selecteddata from three of the alloys in the study. The detrimental effect of0.03% Cu, and the beneficial effect of 0.05% Cr are clearly apparent.

EXAMPLE 4

[0059] AA6060 alloy ingots were produced in the laboratory byconventional DC casting with copper contents of 0.001, 0.006, 0.012,0.016 and 0.03 wt %. The base alloy composition was 0.40 wt % Mg -0.44wt % Si -0.20 wt % Fe -0.007 wt % Zn. The material was extruded, forcedair quenched at the press and aged for 5 hrs@185° C. Samples were etchedin a long life type etchant for incremental times up to 20 minutes. Thegloss values were measured and the samples were weighed to give a valueof metal removal rate.

[0060]FIG. 8 shows the gloss level achieved with a typical metal removalfigure of 100 g/m² as a function of copper content. The gloss levelincreases linearly with copper content within and beyond the inventiverange. FIG. 9 shows the effect of copper content on the metal removalrate. The rate increases slightly above 0.001 wt % Cu but then levelsoff within the defined range before increasing again above 0.016 wt %.The lower metal removal rate associated with 0.015 wt % Cu or less is auseful feature as it means for a given etch time these alloys willundergo less aluminium dissolution and will therefore generate less etchsludge. The same data is presented in FIG. 8 as gloss vs. Metal removal.From this figure it is clear that the alloys within the defined rangeare more efficient in achieving a required gloss level. For example toachieve a gloss level of 80, less metal has to be removed for alloyscontaining <0.016 wt % as compared to the alloy containing 0.03 wt %.

EXAMPLE 5

[0061] Plant Trial

[0062] The following compositions were given identical homogenisationpractices and extruded into the same profile:

[0063] Control:

[0064] 0.46 wt % Si—0.20 wt % Fe—0.04 wt % Cu—0.39 wt % Mg—0.02 wt % Zn

[0065] Alloy within inventive range:

[0066] 0.46 wt % Si—0.20 wt % Fe—0.01 wt % Cu—0.40 wt % Mg—0.02 wt % Zn.

[0067] Some material was left in the T4 temper and the remainder wasaged to the T5 temper (6 hrs@185° C.). Lengths from both compositionsand both tempers were etched and anodised within the same batch. Thematerial was etched for 8 minutes to give a metal removal figure of 104g/m². Some lengths were anodised to give a 5 micron anodic film. Sampleswere also given a 15 minute etch in the laboratory to give 130 g/m²metal removal. FIG. 11 summarises the gloss results. The low copperversion consistently gave a lower gloss finish for both tempers andetches in the as etched and etched plus anodised conditions. These glosstrends corresponded to the visual appearance of the profiles.

EXAMPLE 6

[0068] Plant Trial

[0069] Two compositions were processed in this test

[0070] Control:

[0071] 0.49 wt % Mg—0.47 wt % Si—0.23 wt % Fe—0.004 wt % Zn—0.02 wt % Cu

[0072] Alloy within inventive range:

[0073] 0.38 wt % Mg—0.45 wt % Si—0.20 wt % Fe—0.015 wt % Zn—0.01 wt % Cu

[0074] The two billets were extruded into the same profile underidentical conditions and aged to the T6 temper. FIG. 12 shows the asetched gloss results obtained. The low copper variant gave aconsiderably lower gloss level in spite of the slightly lower Fecontent, which is known to influence the final gloss achieved.

EXAMPLE 7

[0075] Effect of Ageing Practice

[0076] Laboratory tests have demonstrated that the final gloss levelachieved is a very strong function of the ageing practice applied to thealloy. FIG. 13 shows the variation in etched gloss with tensile strengthfor a number of ageing temperatures. The various tensile strength valuesrepresent different heat treatment times at the various temperatures.The results indicate that the lower gloss values are achieved by ageingto full strength at 170 or 185° C. The results also explain some of thevariability in the prior art on this subject.

COMPARATIVE EXAMPLE

[0077] Over a two-year period, 1242 casts of a variant of AA6060 weremade from virgin smelter metal and recycled scrap. The AA6060specification calls for a maximum of 0.10% Cu. When the Cu wascontrolled within the AA6060 specification, the following variation wasfound: Cu Content % Casts with % Casts with Year Number of Casts <0.010%Cu <0.015% Cu 1 786 19.2% 44.4% 2 456 16.9% 45.8%

[0078] The variation in the Cu level in this population of billets isoutside the present invention. Satisfactory extrusion and anodisingperformance was obtained but because of the variation in Cu level fromone cast to another, it was not possible to reduce the amount of metalremoved during etching and still obtain a uniform gloss level.

[0079] By controlling the purity of the virgin metal and the quality andamount or recycled scrap added to each cast, it is possible according tothe invention to reduce the Cu level of all casts below 0.015 or 0.010%Cu to meet a tighter specification within the M6060 composition.

1. A population of billets resulting from more than one cast of metalhaving a specification such that every billet has a composition (in wt%): Constituent Range Preferred Fe <0.35 0.16-0.35 Si 0.20-0.6 0.4-0.6Mn <0.10  0.01-0.05 Mg 0.25-0.9 0.35-0.6  Cu <0.015 <0.010 Ti <0.10 <0.05  Cr <0.10  <0.09  Zn <0.03  <0.03 


2. A billet taken from the population of billets of claim
 1. 3. A methodof making an extruded section by extruding the billet according to claim2.
 4. A method as claimed in claim 3, wherein the extruded section isaged by heating at 1500-200° C. for a time to develop peak strength. 5.A method as claimed in claim 3 or claim 4, wherein the extruded sectionis etched and anodised.
 6. An extruded section made by the method of anyone of claims 3 to
 5. 7. A method of making a population of billets byperforming more than one cast of metal having a specification such thatevery billet has the composition set out in claim 1.